The presently disclosed subject matter relates to method and apparatus for measuring a blood volume ejected by heart pulsation.
Monitoring variations in hemodynamics of a patient in an operating room, an intensive care unit, an emergency treatment room, or a dialysis treatment room within a healthcare center must be performed continuously with minimum possible interruption. Such monitoring of variations in hemodynamics of the patient has hitherto been practiced primarily by means of directly monitoring blood pressure.
A living body, however, usually controls a cardiac output and vascular resistance such that a blood pressure of a nerve center falls within a certain range. Consequently, in order to ascertain variations in hemodynamics of the patient at an early stage, only direct monitoring of a blood pressure is not sufficient. Further, when a change in blood pressure occurs, a cause of the change must be ascertained. For these reasons, in addition to monitoring blood pressure, monitoring a cardiac output is necessary. A known method for measuring a cardiac output includes a thermodilution method, a die dilution method, and an ultrasonic method.
Incidentally, any of the methods described above cannot be readily performed without interruption in light of requirements for sophisticated skills of a healthcare worker and a high degree of invasion of a patient. Under the methods, it has been difficult to continuously monitor variations in hemodynamics of the patient at all times.
On the contrary, Japanese Patent No. 4742644 discloses a blood volume measurement method that enables noninvasive, continuously monitoring of variations in hemodynamics of the patient without interruption, that obviates a necessity for sophisticated skills of a healthcare worker, such as insertion of a catheter, and that assuages a patient's pain. According to the blood volume measurement method, a cardiac output is calculated by utilization of a correlation between a pulse wave transit time and a stroke volume.
However, under the blood volume measurement method described in Japanese Patent No. 4742644, blood pressure must be measured at least twice or more on the occasion of determination of a coefficient α inherent in the patient, which results in an increase in measurement time and may impose unwanted stress on the patient. Further, when a difference does not exist between two or more measured values of blood pressure, calibration of α0 and β cannot be practiced. For this reason, blood pressure must be measured while the patient is put under load in order to make values of blood pressure differ from each other; hence, measuring values of blood pressure itself is not easy.